The measurement and detection of components of gases, particularly ambient air, are important in environmental monitoring, forensics, and as a research tool.
In one simple technique, a colorimetric indicator containing a specific solid that absorbs components from a gas and reacts with it, qualitatively detects the presence of certain analytes by producing a characteristic color change. For example, indicator devices are commercially available that detect the presence of toxic chemicals such as carbon monoxide and hydrogen sulfide. Although such calorimetric devices are usually inexpensive, they are not useful in the detection of gas components generally because they are limited by the reactivity of the solid indicator material for the complementary specific chemicals for which they were designed. Furthermore, colorimetric indicators are generally not capable of quantitative measurement.
Other techniques of more general utility permit the detection and measurement of components of gas samples, including low concentration components. Frequently, the detection of low concentration components of gases is limited by the sensitivity of instruments. In order to improve low concentration measurements, gas-borne analytes may be concentrated prior to detection. Typically, a three step process is used to study the components of a gas sample: collection of the gas sample, pre-concentration of the analytes therein, and detection of the analytes. For example, a gas sample may be collected by an evacuated canister of about 6 to 12 liters (or larger) that, upon opening a control valve, sucks in ambient air. The ambient air is then returned to the laboratory for analysis. See, e.g., McClenny, et al., “Canister-based method for monitoring toxic VOCs in ambient air,” J. Air Waste Manage. Assoc. 41, 1308-18 (1991). More conveniently, however, the steps of collection of the gas sample, and pre-concentration and detection of analytes are often performed simultaneously.
One method of pre-concentrating analytes from the gaseous phase involves passing a sample gas through a liquid suitable for dissolving the analytes. For example, a pollutant gas may be collected by bubbling it through a liquid in which the pollutant dissolves. Typically, ambient air enters an extraction device through a moisture trap and filter, and then it is contacted with a liquid extraction medium, which is then analyzed to determine the concentration of gaseous pollutant. When used with continuous-feed mechanisms, large gas volumes may be extracted and analyzed. Gas-liquid extraction methods are not widely used because they typically require large volumes of solvent, expensive machinery, and lengthy analysis times.
A more common method of pre-measurement concentration uses “adsorption” (or “diffusion”) tubes containing a “sorbent” material, e.g., activated charcoal, that selectivity retains and desorbs toxic chemicals, contaminants, or other substances of interest from a gas sample. A pump may move a gas sample through a pre-concentrator tube containing a sorbent material that retains analytes of interest until a sufficiently large sample accumulates. A heating element wrapped around the pre-concentrator tube is then used to heat the sorbent material and thereby desorb or volatilize the chemicals. Upon desorption, the chemicals are conducted to a measurement instrument that indicates the presence, identity, or amount of the various chemicals.
Certain solids are capable of retaining specific analytes on their surface. Activated carbon or charcoal is often used to retain and concentrate organic chemicals from air streams. Various other sorbent materials may also be used, depending on the nature of the analytes of interest. For example, one material useful in the analysis of amines includes a mixture of an adsorbent organic polymer having an affinity for amines, e.g., Tenax® (2,6-diphenylphenylene oxide polymer, a registered trademark of Buchem, N.V.), and a second adsorbent that is an inorganic porous material and is alkaline, e.g., soda lime. U.S. Pat. No. 4,701,306.
Typically, sorbent-based techniques are solid phase extraction methods that use a flow-through chamber containing a stationary phase material to preferentially retain components of interest from a gas sample flushed through the cartridge. A solvent in which the retained analytes are soluble is then flushed through the cartridge, thereby producing a solution of components of interest for analysis. Several adsorption tubes are commercially available, including those filled with a variety of stationary phase adsorbent materials such as activated carbon, graphitized carbon black, silica gel, quartz glass, carbon molecular sieves, or poly(2,6-diphenylphenylene oxide). Although solvent-wetted sorbent materials have also been used to pre-concentrate analytes from vapors or aerosols, U.S. Pat. Nos. 5,173,264, 4,912,051, and 4,977,095, dry sorbent materials are more commonly used.
EPA Methods TO-1, -5, and -17, for example, are sorbent-based methods for the measurement of volatile organic compounds in ambient air. In these EPA methods, a sample is passed through a cartridge containing an adsorbent such as Tenax®, XAD-2 (a trademark of Rohm and Haas Co. Corp. of Philadelphia, Pa.), or charcoal, and then analytes are thermally desorbed or solvent extracted for analysis. See Riggin, “Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air: Methods TO-1 and TO-2,” U.S. Environmental Protection Agency, Research Triangle Park, N.C. 27711, EPA 600/4-84-041, April 1984; Winberry, et al., “Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air: Method TO-14, Second Supplement,” U.S. Environmental Protection Agency, Research Triangle Park, N.C. 27711, EPA 600/4-89-018, March 1989; Anon., “Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air, Second Edition: Method TO-17,” U.S. Environmental Protection Agency, Cincinnati, Ohio 45268, EPA/625/R-96/010b, January 1997. See also, Woolfenden, “Monitoring VOCs in air using sorbent tubes followed by thermal desorption-capillary GC analysis—Summary of data and practical guidelines,” J. Air Waste Manage. Assoc. 47, 20-36 (1997); Pankow, et al., “Determination of a wide range of volatile organic compounds in ambient air using multisorbent adsorption/thermal desorption and gas chromatography/mass spectrometry,” Anal. Chem. 70, 5213-21 (1998); Ciccioli, et al., “GC Evaluation of the Organic Components Present in the Atmosphere at Trace Levels with the Aid of Carbopack B for Preconcentration of the Sample,” J. Chrom. 351, 433-49 (1986); Brown, et al., “Collection and Analysis of Trace Organic Vapour Pollutants in Ambient Atmospheres: The Performance of a Tenax-GC Adsorbent Tube,” J. Chrom. 178, 79-90 (1979); Walling, “The Utility of Distributed Air Volume Sets When Sampling Ambient Air Using Solid Adsorbents,” Atmos. Environ. 18, 855-59 (1984); and U.S. Pat. No. 4,541,268.
When used in conjunction with gas chromatography, an absorption tube is heated to rapidly desorb the retained analytes and produce a concentrated analyte pulse that is injected into a gas chromatography column. In such a method, the pre-concentrator tube serves a dual purpose of sample pre-concentration and injection. The different components in the sample are then chromatographically separated and analyzed by a detector. Gas chromatography is one common detection and measurement technique in which a gas sample containing analytes of interest is introduced into a flowing stream of a carrier gas and pumped through a capillary, the wall of which is coated with a stationary phase material. The analytes partition between the moving carrier gas and the stationary phase by absorbing and desorbing from the coating as they migrate through the capillary. The components are thus separated according to their relative partition coefficients and they exit the capillary at different times. Analytes with similar physical and chemical properties may thus be efficiently separated. A detector that is chosen based on the type of gases being analyzed is positioned at the outlet of the capillary and constantly measures the gas mixture as it exits. Exemplary detectors include thermal conductivity, electron capture, flame ionization, mass spectrometry, and chemiluminescence detectors.
Sorbent-based gaseous analyte pre-concentration methods have several disadvantageous features. For example, low gas permeability of sorbent materials requires sampling of high gas volumes. The sorbent material may be inadvertently overloaded. If the sorbent is highly retentive of an adsorbed trace material, then it will require relatively large elution volumes. Also, the analyte may react with the sorbent material or the material itself may cause introduction of interfering contaminants that compromise the analytical data. There is the added disadvantage of the time required to regenerate (or replace and condition) the chemical sorbent or membrane, and carry out the gas-chromatographic separation prior to detection, which limits output. Unless the pre-concentrator tube is heated for a sufficiently long time, all of the accumulated analytes may not be released, and the sample of chemical released from the sorbent material that reaches the detector may not accurately reflect the concentration of chemical entering the sorbent material. Furthermore, the sorbent may exhibit a memory effect in which chemicals remaining in the sorbent material are released when the pre-concentrator tube is heated a subsequent time, thereby producing artificially higher measurements. In order to measure analytes present in concentrations below about 1 ppb in air, a relatively large volume of sorbent, which requires large elution volumes, may be required, and in such cases some substances (particularly non-volatile trace materials) that bind very strongly may be difficult to desorb.
As an alternative to sorbent-based pre-concentration methods, “cryofocusing” (or “thermofocusing”) may be used to pre-concentrate analytes from a gas sample in a thermal focusing chamber or cold trap. Cryofocusing is the concentration of analytes by condensing them out of their original gas matrix into a smaller volume in an inert tube. A typical cold trap is a capillary tube enveloped by a cold vessel that is chilled. In such a device, a gas sample is passed through the capillary and, by exposing incoming analyte to the low temperatures, it condenses within the capillary. When sufficient amounts of analyte have been accumulated, the temperature of the capillary passing through the cold trap is rapidly increased to volatilize the sample. A carrier gas stream that continually flows through the trap then injects the analyte into the column for separation. Pankow, et al., “Determination of volatile compounds in water by purging directly to a capillary column with whole column cryotrapping,” Environ. Sci. Technol. 22, 398-405 (1988); U.S. Pat. Nos. 5,005,399, 5,595,709, 5,795,368, 5,954,860, 5,872,306; Ewels, et al., “Electrically Heated Cold Trap Inlet System for High-Speed Gas Chromatography,” Anal. Chem. 57, 2774-79 (1985); Mouradian, et al., “Evaluation of a Nitrogen-Cooled, Electrically Heated Cold Trap Inlet for High-Speed Gas Chromatography,” J. Chrom. Sci. 28, 643-48 (1990); Klemp, et al., “Cryofocusing Inlet with Reverse Flow Sample Collection for Gas Chromatography,” Anal. Chem. 65, 2516-21 (1993); Tijssen, et al., “Theoretical Aspects and Practical Potentials of Rapid Gas Analysis in Capillary Gas Chromatography,” Anal. Chem. 59, 1007-15 (1987); and Giese, et al., “Adsorption/thermal desorption for the determination of volatile organic compounds in water,” J. Chrom. 537, 321-28 (1991).
In the past, cryofocusing has typically been accomplished by cooling the sample gas stream to liquid nitrogen temperatures. The analytes can then be transferred to the column by flash heating. Often, the sample is then “refocused” at the head of the column. By combining cryofocusing methods with sorbent-based techniques in a trap, a similar effect can be achieved with trapping at ambient temperatures.
Although some of the shortcomings discussed above respecting sorbent-based pre-concentration methods are overcome by thermal focusing techniques, a number of disadvantages persist. As a practical matter, cryogenic methods of pre-concentration are limited by small sample volumes (about 10 mL or less). Also, the use of thermal focusing with a gas chromatography system produces undesirable dead volume and band broadening which adversely affect system resolution and efficiency. After heating the cold trap sample tube, the sample components must traverse the entire length of the sample tube before introduction into the column. As the length of time that the sample is injected at the inlet end of the column increases, the peaks produced by elution of the components tend to broaden.
The requirement of application of heat in both sorbent-based pre-concentration methods and thermal focusing methods is a significant limitation. Certain compounds thermally degrade upon heating at temperatures required for thermal desorption or GC separations. Therefore, these methods are not ideally used in the analysis of thermal sensitive compounds. In GC thermal focusing, analytes may be exposed to excessive temperatures in order to volatilize them entirely through the focusing chamber, thereby causing thermal decomposition. Instead of actual analytes of the original sample being ejected from the column, these components become fragmented, which significantly complicates analysis. Furthermore, the entire length of the cold trap sample tube cannot be maintained ideally at a uniform constant temperature, either during the collection or injection modes, and a thermal gradient exists. Because during the collection mode of operation, the analyte condenses near the inlet end of the capillary tube, making it necessary to insure that region is sufficiently heated to volatilize all of the component during the injection step. This requirement leads to some portions of the cold trap sample tube being heated to a significantly higher temperature than is necessary to volatilize the sample collected.